This invention relates to the ceramic arts and, more particularly, it relates to a method for chemical vapor deposition (CVD) of silicon carbide (SiC) coatings onto ceramic substrates.
In the art, coatings of SiC by CVD have been applied to nuclear fuel particles to provide structural integrity, radiation resistance, and fission product retention upon exposure to intense radiation within a reactor environment. Typical fuel coatings comprise an inner layer of pyrolytic carbon (PyC), a dense middle layer of PyC, and a dense outer layer of SiC. If desired, additional coatings of PyC, SiC, Al.sub.2 O.sub.3 and similar refractory compounds may be applied for greater mechanical strength and reactor environment inertness.
Customarily, these coating operations are conducted in fluidized-bed coating units during batch-wise operations. Gaseous feeds thereto include appropriate donor gases, inert gases, and carbon sources that provide a desired coating layer through CVD. Pyrolytic carbon coatings are applied first to nuclear fuels at about 1000.degree. C. to 1200.degree. C. Thereafter, SiC coatings are applied within a temperature range of about 1500.degree. C. to 1600.degree. C. using methyltrichlorosilane (MTS) as the SiC donor gas and hydrogen as the fluidizing gas. Corrosive gases such HCl, Cl.sub.2, and the like are evolved upon thermal decomposition of the MTS.
Recently, the above-described technology has been applied to nuclear waste forms derived by sol-gel type operations to effect multi-barrier consolidations of high-level radioactive wastes. Precise details for such procedures may be found in a commonly assigned U.S. patent application entitled "Method For Fixation and Containment of Nuclear Wastes", Ser. No. 217,359 deposited Dec. 17, 1980 in the names of Peter Angelini, et al. The pertinent contents of that patent application have also been published in "Sol-Gel Technology Applied To Glass and Crystalline Ceramics", Waste Management Symposium 1980, Vol. 2, pp. 391-417, Tucson, Ariz. (Mar. 9-14, 1980). Inasmuch as these teachings may be utilized to provide a solidified waste form which can be employed in the method of the present invention to achieve significant improvements in the requisite chemical and mechanical properties for an ultimate waste form containing high level radioactive wastes, the foregoing patent application and publication are incorporated herein by reference.
While it may appear from the aforementioned teachings that prior experience with coated nuclear fuels may be applied to solidified forms of nuclear waste to achieve CVD coatings, the unpredictable behavior of nuclear wastes makes such an adaption difficult. In actual practice, the complex compositions and multi-phase systems of nuclear wastes respond quite differently to thermal and environmental conditions customarily employed for coating the relatively pure phase systems of one or two metal constituents present in nuclear fuels. For example, nuclear waste particles melt several hundreds of degrees lower than nuclear fuel materials and are usually molten at the typical PyC (1300.degree. C.) or SiC (1550.degree. C.) deposition temperatures utilized for fuel coatings.
Another problem associated with coating nuclear waste has been the inability to achieve large volumetric reductions by producing forms having high concentrations. An even more troublesome problem has been the adverse reactions between contents and components of the coating unit, including the nuclear waste particles, and the decomposition products of gases utilized in fuel fabrication technology. Finally, the application of fuel coatings to nuclear waste forms have produced less than satisfactory leach resistance. Based upon prior experience, the leach resistance of coated nuclear wastes is about two orders of magnitude lower and is the direct result of the aforemetioned problems.
FIG. 1 is a photograph of a typical fuel-type coated nuclear waste containing about 90% nuclear waste. As is apparent therein, deleterious reactions have taken place within the kernel and between the kernel and the PyC coating during the coating process causing voids, surface abnormalities, and structural distortions. These defects may have a deleterious effect on integrity and long-term service life of the particles. For example, the core area of the structures does not appear uniform indicating melting and possible loss of radioactive species by mechanisms such as volatilization or migration into adjoining layers. Since the distorted shape and penetrated coating layers visible in FIG. 1 may provide potential pathways for subsequent chemical or physical damage, the suitability of these particles as an ultimate solidified nuclear wastes is doubtful. It has also been found that other substrates, of which nuclear waste forms are but one species, are also incompatible with known CVD technology for nuclear fuels for the reasons given above.
Accordingly, it is desired to develop a method for coating substrates to provide an essentially insoluble and inert barrier of nonreactive refractory compounds which is compatible therewith while maintaining the substrate in a stable condition. It is also contemplated that the resultant coated substrate will possess a significant improvement in chemical and mechanical properties.
As used herein, the term "substrate" shall include solidified nuclear waste particles derived by sol-gel type and other waste solidification processes as well as more conventional articles to which the application of SiC coatings by CVD may appear desirable.
Other substrates to which the method of the present invention may be applied to provide SiC coatings include valve seats and bonnets, pump impellers, working surfaces of advanced power plants, and wear surfaces in equipment for coal liquefaction and conversion processes.